1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an active matrix organic electroluminescent device including amorphous silicon thin film transistor and a fabricating method thereof.
2. Discussion of the Related Art
In general, an organic electroluminescent device (ELD) emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Contrary to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the exciton between states causes light to be emitted. Accordingly, the size and weight of the organic ELD can be reduced. The organic ELD has other excellent characteristics such as low power consumption, superior brightness, and fast response time. Because of these characteristics, the organic ELD is regarded as a promising candidate for next-generation consumer electronic applications such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is a simple process with few processing steps, it is much cheaper to produce an organic ELD than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabricating process, the passive matrix organic ELD requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic ELDs are highly efficient and can produce a high-quality image for a large display with relatively low power.
FIG. 1 is a schematic cross-sectional view of an organic ELD according to the related art. In FIG. 1, an array element 14 including a thin film transistor (TFT) “T” is formed on a first substrate 12. A first electrode 16, an organic electroluminescent layer 18, and a second electrode 20 are formed over the array element 14. The organic electroluminescent layer 18 may separately display red, green, and blue colors for each pixel region. Generally, separate organic materials are used to emit light of each color for the organic electroluminescent layer in each pixel region. An organic ELD is encapsulated by attaching the first substrate 12 and a second substrate 28, which includes a moisture absorbent material 22, with a sealant 26. The moisture absorbent material 22 eliminates moisture and oxygen that may penetrate into a capsule of the organic electroluminescent layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent material 22 and the filled moisture absorbent material is fixed by a holding element 25.
FIG. 2 is an equivalent circuit diagram of an organic electroluminescent device according to the related art. In FIG. 2, a gate line 14 crosses a data line 16, and a switching element “TS” is connected to the gate line 14 and the data line 16 at the crossing point of the gate line 14 and the data line 16. A driving element “TD” is electrically connected to the switching element “TS” and an organic electroluminescent diode “DEL.” A storage capacitor “CST” is formed between a driving gate electrode 20 and a driving drain electrode of the driving element “TD,” and the organic electroluminescent diode “DEL” is connected to a power line 22.
When a scan signal of the gate line 14 is applied to a switching gate electrode 18 of the switching element “TS,” an image signal of the data line 16 is applied to the driving gate electrode 20 of the driving element “TD” through the switching element “TS.” The current density of the driving element “TD” is modulated by the image signal applied to the driving gate electrode 20. As a result, the organic electroluminescent diode “DEL” can display images with gray scale. Moreover, since the image signal stored in the storage capacitor “CST” is applied to the driving gate electrode 20, the current density flowing into the organic electroluminescent diode “DEL” is kept uniform until the next image signal is applied even when the switching element “TS” is turned OFF. The switching element “TS” and the driving element “TD” can be formed of a polycrystalline silicon TFT or an amorphous silicon TFT. The process of fabricating an amorphous silicon TFT is simpler than the process for a polycrystalline silicon TFT. The amorphous silicon TFT should have a larger width to length ratio (W/L ratio) to drive the organic electroluminescent diode “DEL.” As the W/L ratio of the amorphous silicon TFT becomes larger, the current density flowing through the amorphous silicon TFT increases. High current density may cause the amorphous silicon TFT to degrade due to stress, thereby disadvantageously changing conductive characteristics of the amorphous silicon TFT. Moreover, the changes in the conductive characteristics of the amorphous silicon TFT are exacerbated when a direct current (DC) bias is continuously applied to the driving element “TD” in the organic electroluminescent device. As a result, the display quality of an amorphous silicon TFT is inferior and may result in residual images being displayed. In addition, the driving element “TD” may sometimes break down due to the stress caused by the increased current density. When the driving element “TD” is composed of one TFT, a broken TFT would cause a point defect.